Operation of falling film evaporator involving viscous liquid

In this thesis, the working principles of falling film evaporator used in dairy industries in relation to the evaporation of skim milk have been explored with a focus on the effect of milk solids content. The changes in rheological behaviour of skim milk and heat transfer within the evaporator during the concentration process have been investigated. With the better understanding of the rheological behaviour of skim milk and the operation of a falling film evaporator, the possibility of improving the performance of commercial falling film evaporator from its current configuration can be assessed. The use of falling film evaporator is a common and economical practice in the food and beverage industries to remove water from liquid products, e.g. juices, milk, etc., especially when the product is temperature sensitive. In dairy industries, falling film evaporators are used to evaporate water from dairy products such as milk, protein concentrate, etc. Falling film evaporator is a well established technology but the understanding of rheological influence of milk on the performance and operation of falling film evaporator remains relatively unexplored which formed the motivation of this project. In milk powder production, milk is normally concentrated from around 10wt% to 50wt% or 52wt% (this depends on the type of milk) using multi-effect falling film evaporator prior to the spray drying process. The viscosity changes of milk during the concentration process are evident, especially the exponential rise in viscosity as solids level reaches 50wt%. With the increase in viscosity, the flowing characteristics of the milks within the tubes in the evaporator change with the concentration of milk. Consequently, the performance of the evaporator, in terms of heat transfer, is considerably affected. The current research project has been divided into 3 main parts into context of viscosity and evaporation of milk using falling film evaporator. Firstly, viscosity models of milk are established based on stringent viscosity measurement procedures that ensure the repeatability and reliability of the measurements. These models are used for process simulation, model based control and production planning. The second part is to evaluate the performance of falling film evaporator under the influences of various operation conditions. In the last part, findings from the first two sections are merged into a pilot evaporator mathematical model that is able to predict the solid content, flow characteristics and residence time of milk concentrate in a falling film evaporator. The viscosity of milk is influenced by several factors such as solids content, temperature, ageing and shearing, etc. The focus in this thesis is on the instantaneous milk viscosity during the operation of the falling film evaporator. Therefore, viscosity measurements and modelling of the milk viscosity have been mainly focused on only on the effect of solids content, temperature and, to some extend, shear rate. A 2m steam-heated pilot evaporator was designed and constructed to commercial grade based on a falling film evaporator design. This pilot evaporator is able to operate under vacuum conditions (up to -85kPa gauge) in both shell and tube side so as to mimic the operation conditions in the dairy industries. The viscosity models in this thesis are predominately formulated based on the fresh and reconstituted medium heat-treated skim milk, unless otherwise stated. A comparison of viscosity models were made between the reconstituted and fresh skim milk. Significant differences were found and reported between the two types of milks. The heat transfer coefficient (HTC), a common method of quantifying the performance of an evaporator, was measured based on temperature difference between the heat transfer surface and the processed fluid (e.g. milk) and the energy transfer within the evaporator. Another 1m electric-heated pilot evaporator was designed and built to facilitate the measurement at such temperatures and to have known power inputs which was not accessible in the steam-heated device. The influence of different operating conditions, such as varying heat flux, flow characteristics (e.g. Re number) and protein content, on the heat transfer coefficient is thoroughly investigated in this thesis. The study into the HTC on the product side found that the HTC measured from the evaporation of reconstituted skim milk is unresponsive to some of the operating conditions it was subjected to as compared to the fresh skim milk. Generally, HTC improves with greater Re number, heat flux and protein content (milk protein concentrate was added). Visual examination of the evaporation conditions within the evaporator also indicates that the amount of bubble formed during the evaporation process appears to increase with increasing flow rate, heat flux and protein content. During the formulation of the pilot evaporator model in this thesis, the viscosity models and heat transfer coefficients established in the current project were incorporated so as to model the steam-heated batch pilot evaporator. Several assumptions were introduced in order to create a working model. The pilot evaporator model has been verified against the actual experimental data and is proven to be accurate. This model is able to predict the solids content of the skim milk at any given time provided that the operating conditions are available. This model can also be applied onto a commercial falling film evaporator with minor modification to the calculation sequence. A scale-up version has been developed but due to commercial nature, it is not reported in this thesis. The research into the operation of falling film evaporator has been seen to enhance the knowledge of the mechanisms and interactions between the process fluid and the evaporator. Some of the results obtained here have already benefited an industrial operation.

本论文围绕乳品工业中用于脱脂乳（skim milk）浓缩的降膜蒸发器（falling film evaporator）工作原理展开研究，重点聚焦乳固体含量的影响。本研究考察了浓缩过程中脱脂乳的流变特性变化与降膜蒸发器内的传热行为。通过加深对脱脂乳流变特性及降膜蒸发器运行机制的理解，可评估现有商用降膜蒸发器在当前配置下的性能优化潜力。

降膜蒸发器是食品饮料工业中脱除液态物料水分的通用且经济的技术手段，尤其适用于热敏性物料，如果汁、牛乳等。在乳品工业中，降膜蒸发器常用于脱除牛乳、蛋白浓缩物等乳制品中的水分。尽管降膜蒸发器是一项成熟的技术，但学界对乳的流变特性如何影响降膜蒸发器的性能与运行的认知仍相对匮乏，这也正是本项目的研究动机。

在乳粉生产中，通常先通过多效降膜蒸发器（multi-effect falling film evaporator）将牛乳从约10%（质量分数，wt%）浓缩至50%或52%（具体取决于牛乳品类），随后再进行喷雾干燥（spray drying）。浓缩过程中牛乳的黏度变化十分显著，尤其是当固体含量达到50%时，黏度会呈指数级上升。随着黏度升高，蒸发器管内牛乳的流动特性随乳固体浓度变化而改变，进而显著影响蒸发器的传热性能。

本研究围绕降膜蒸发器下的牛乳黏度与蒸发过程，主要分为三大模块：其一，基于严格的黏度测量流程建立牛乳黏度模型，确保测量结果的可重复性与可靠性，该模型可用于过程模拟、基于模型的控制及生产规划；其二，评估不同运行条件对降膜蒸发器性能的影响；其三，将前两模块的研究结果整合为中试蒸发器（pilot evaporator）数学模型，该模型可预测降膜蒸发器内脱脂乳浓缩液的固体含量、流动特性与停留时间。

牛乳黏度受多种因素影响，包括固体含量、温度、陈化时间与剪切作用等。本论文聚焦降膜蒸发器运行过程中脱脂乳的瞬时黏度，因此黏度测量与建模主要围绕固体含量、温度，并在一定程度上考虑剪切速率的影响。

本研究设计并搭建了一台符合商用级标准的2m蒸汽加热型中试蒸发器，该设备可在壳程与管程均实现真空工况（最高表压-85kPa），以模拟乳品工业中的实际运行环境。除非特别说明，本论文中的黏度模型均基于新鲜及复原型中热处理脱脂乳建立，并对比了复原与新鲜脱脂乳的黏度模型，发现二者存在显著差异并进行了报道。

传热系数（heat transfer coefficient, HTC）是量化蒸发器性能的常用指标，其测量基于传热表面与加工流体（如牛乳）间的温差及蒸发器内的能量传递。为实现在特定温度下的测量并获得可控的输入功率（蒸汽加热设备无法实现该条件），本研究还设计搭建了一台1m电加热型中试蒸发器。本论文深入考察了不同运行条件——包括变化的热通量、流动特性（如雷诺数（Re number））与蛋白质含量——对传热系数的影响。针对产品侧传热系数的研究发现，与新鲜脱脂乳相比，复原脱脂乳蒸发过程中测得的HTC对部分受试运行条件无明显响应。总体而言，HTC随Re数、热通量与蛋白质含量（添加牛乳蛋白浓缩物后）升高而提升。对蒸发器内蒸发工况的可视化观察也表明，蒸发过程中产生的气泡量随流速、热通量与蛋白质含量的增加而增多。

本论文在构建中试蒸发器模型时，整合了本项目建立的黏度模型与传热系数，以模拟蒸汽加热型间歇式中试蒸发器。为构建可用模型，本研究引入了若干假设。该中试蒸发器模型已通过实际实验数据验证，证明具备较高精度。只要给定运行条件，该模型即可预测任意时刻脱脂乳的固体含量，且经少量计算序列修改后，可应用于商用降膜蒸发器。本研究还开发了缩放版模型，但鉴于商业机密性，未在论文中公开。

本研究对降膜蒸发器运行机制的探索，加深了对加工流体与蒸发器间相互作用机制的认知。本研究获得的部分成果已助力工业生产运行。